The main goal of this project is to define the portion of the CD4 molecule which is used as the receptor for human immunodeficiency virus (HIV). CD4 is a T lymphocyte differentiation antigen which defines the subset of human T lymphocytes involved in recognition of class II major histocompatibility complex molecules. This subset includes most T cells with helper and/or inducer activity and is the subset which is depleted in patients with human acquired immune deficiency syndrome (AIDS). HIV is the retrovirus responsible for AIDS, and this virus uses the CD4 molecule as its receptor. In order to define the region(s) of the CD4 molecule involved in HIV receptor activity, we will take advantage of the fact that L3T4, the mouse homolog of human CD4, does not bind HIV. Hybrid constructs will be made using portions of the mouse and human cDNAs or genes. These hybrid constructs will be transfected into human cells, where they should result in cell surface expression of hybrid CD4- L3T4 protein. These transfectants will be assayed for their ability to bind the virus and to be infected by it. The important functional region(s) will be further defined by use of synthetic peptides and site-specific mutagenesis to prevent binding and infection. It has also been observed that human CD4 does not make mouse cells susceptible to infection by HIV, although mouse cells expressing human CD4 do bind the virus. This may be a result of inability of a portion of human CD4 distinct from the binding site to interact with mouse molecules necessary for productive virus entry into the cell. To explore this the mouse- human hybrid constructs will also be assayed in mouse cells to determine whether any of the resultant hybrid proteins can lead to infection of mouse cells. The possibility of additional human cellular components being necessary for infection will also be analyzed by fusion of non-susceptible CD4+ transfected mouse cells to CD4- human cells and determination of whether the cell hybrids can be infected by HIV. These studies are important for an understanding the mechanism of HIV entry into the cell and may lead to new ideas concerning means of prevention of infection.